Malaria, which is caused by infection with the parasite Plasmodium falciparum (PF), represents a major world health problem. Approximately 500 million people in the world are at risk from the disease, with approximately 200 million people actually harboring the parasites. An estimated 1 to 2 million deaths occur each year due to malaria. (Miller et al., Science 234:1349, 1986).
Fatal outcomes are not confined to first infections, and constant exposure is apparently a prerequisite for maintaining immunity. Naturally acquired sterile immunity is rare, if it exists at all. Accordingly, major efforts to develop an efficacious malaria vaccine have been undertaken.
Human volunteers injected with irradiated PF sporozoites are resistant to subsequent sporozoite challenges, which demonstrates that development of a malaria vaccine is indeed immunologically feasible. Furthermore, these immune individuals developed a vigorous response, including antibodies, and cytotoxic T lymphocyte (CTL) and helper T lymphocyte (HTL) components, directed against multiple antigens. Reproducing the breadth and multiplicity of this response in a vaccine, however, is a task of large proportions. The epitope approach, as described herein, may represent a solution to this challenge, in that it allows the incorporation of various antibody, CTL and HTL epitopes, from various proteins, in a single vaccine composition.
Anti-sporozoite antibodies are by themselves, in general, not completely efficacious in clearing the infection (Egan et al., Science 236:453, 1987). However, high concentrations of antibodies directed against the repeated region of the major B cell antigen of the sporozoite/circumsporozoite protein (CSP) have been shown to prevent liver cell infection in certain experimental models (Egan et al., Science 236:453, 1987; Potocnjak, P. et al., Science 207:71, 1980). The present inventors have shown that constructs encompassing CSP-repeat B cell epitopes and the optimized helper epitope PADRE™ (San Diego, Calif.) are highly immunogenic, and can protect in vitro against sporozoite invasion in both mouse and human liver cells, and protect mice in vivo against live sporozoite challenge (Franke et al., Vaccine 17:1201-1205, 1999)
PF-specific CD4+ T cells also have a role in malarial immunity beyond providing help for B cell and CTL responses. Experiments by Renia et al. (Renia, et al., Proc. Natl. Acad. Sci. USA 88:7963, 1991) demonstrated that HTLs directed against the Plasmodium yoelli CS protein could in fact adoptivley transfer protection against malaria.
Considerable data implicate CTLs in protection against pre-erythrocytic-stage malaria. CD8+ CTLs can eliminate Plasmodium berghei- or Plasmodium yoelii-infected mouse hepatocytes from in vitro culture in a major histocompatibility complex (MHC)-restricted and antigen-restricted manner (Hoffman et al., Science 244:1078-1081, 1989; Weiss et al., J. Exp. Med. 171:763-773, 1990). Further, it has also been shown that the immunity that developed in mice vaccinated with irradiated sporozoites is also dependent upon the present of CD8+ T cells. These T cells accumulate in inflammatory liver infiltrates subsequent to challenge. Passive transfer of circumsporozoite (CSP)-specific CTL clones as long as three hours after inoculation of sporozoites (i.e., after the parasites have left the bloodstream and infected liver cells) were capable of protecting animals against infection (Romero et al., Nature 341:323, 1989).
It is notable that CTL-restricted responses directed against a single antigen are insufficient to protect mice with different MHC alleles, and a combination of multiple antigens was required even to protect mice from the most common laboratory strains of Plasmodium. These data indicate that a combination of epitopes form several antigens is necessary to elicit a protective CTL response.
Indirect evidence that CTLs are important in protective immunity against Pf in humans has also accumulated. It has been reported that cytotoxic CD8+ T cells can be identified in humans immunized with PF sporozoites (Moreno, et al., Int. Immunol. 3:997, 1991). Further, humans immunized with irradiated sporozoites or naturally exposed to malaria can generate a CTL response to the pre-erythrocytic-stage antigens, CSP, sporozoite surface protein 2 (SSP2), liver-stage antigen-1 (LSA-1), and exported protein-1 (Exp-1) (see, e.g. Malik et al., Proc. Natl. Acad. Sci. USA 88, 3300-3304, 1991; Doolan et al., Int. Immunol. 3:511-516, 1991; Hill et al., Nature 360:434-439, 1992). Additionally, there is evidence that the polymorphism within the CSP may be the result of selection by CTLs of parasites that express variant forms (MCutchan and Water, Immunol. Lett. 25:23-26, 1990). This is based on the observation that the variation is nonsynonymous at the nucleotide level, thereby indicating selective pressure at the protein level. The polymorphism primarily maps to identified CTL and T helper epitopes (Doolan et al., Int. Immunol. 5:27-46, 1993); and CTL responses to some of the parasite variants do not cross-react (Hill et al., supra). Finally, the MHC class I human leukocyte antigen (HLA)-Bw53 has been associated with resistance to severe malaria in The Gambia, and CTLs to a conserved epitope restricted by the HLA-Bw53 allele have been identified on P. falciparum LSA-1 (Hill et al., Nature 352:595-600, 1991; Hill et al., Nature 340:434-439, 1992). Since HLA-Bw53 is found in 15%-40% of the population of sub-Saharan Africa but in less than 1% of Caucasians and Asians, these data suggest evolutionary selection on the basis of protection against severe malaria.
Thus, antibody, and both HLA class I and class II restricted responses directed against multiple sporozoite antigens appear to be involved in generating protective immunity to malaria. Furthermore, several important antigenic epitopes against which humoral and cellular immunity is focused have already been exactly delineated.
HLA class I molecules are expressed on the surface of almost all nucleated cells. Following intracellular processing of antigens, epitopes from the antigens are presented as a complex with the HLA class I molecules on the surface of such cells. CTL recognize the peptide-HLA class I complex, which then results in the destruction of the cell bearing the HLA-peptide complex directly by the CTL and/or via the activation of non-destructive mechanisms e.g., the production of interferon.
In view of the heterogeneous immune response observed with PF infection, induction of a multi-specific cellular immune response directed simultaneously against multiple PF epitopes appears to be important for the development of an efficacious vaccine against PF. There is a need, however, to establish vaccine embodiments that elicit immune responses that correspond to responses seen in patients that clear PF infection.
The information provided in this section is intended to disclose the presently understood state of the art as of the filing date of the present application. Information is included in this section which was generated subsequent to the priority date of this application. Accordingly, information in this section is not intended, in any way, to delineate the priority date for the invention.